Heretofore, a thermal type flow control device MFC and a pressure-type flow control device FCS have been widely used in a gas supply device for semiconductor control devices. In particular, as illustrated in FIG. 19, the latter pressure-type flow control device FCS is configured from, for example, a control valve CV, a temperature detector T, a pressure detector P, an orifice OL, and a calculation control unit CD containing a temperature correction/flow rate calculation circuit CDa, a comparison circuit CDb, an input/output circuit CDc, an output circuit CDd, and the like and has an outstanding flow rate characteristic which enables stable flow control even when the primary side supply pressure sharply varies.
More specifically, in the pressure-type flow control device FCS of FIG. 19, the detection values from the pressure detector P and the temperature detector T are converted to digital values, and then input into the temperature correction and the flow rate calculation circuit CDa. Herein, the temperature correction and the flow rate calculation of the detected pressure are performed, and then the calculated flow rate value Qt is input into the comparison circuit CDb. An input signal Qs corresponding to a set flow rate is input from a terminal In, converted to a digital value in the input/output circuit CDc, input into the comparison circuit CDb, and then compared with the calculated flow rate value Qt from the temperature correction/flow rate calculation circuit CDa herein. As a result of the comparison, when the set flow rate input signal Qs is larger than the calculated flow rate value Qt, a control signal Pd is output to a driving unit of the control valve CV. Thus, the control valve CV is driven in the closing direction to be driven in the valve closing direction until a difference (Qs-Qt) between the set flow rate input signal Qs and the calculated flow rate value Qt reaches zero.
In the pressure-type flow control device FCS, when a so-called critical expansion condition of P1/P2≧about 2 is held between a downstream side pressure P2 and an upstream side pressure P1 of the orifice OL, the flow rate Q of gas flowing through the orifice OL is Q=KP1 (K is a constant) and the flow rate Q can be controlled with high accuracy by controlling the pressure P1 and also an outstanding characteristic is given in which, even when the pressure of the upstream side gas Go of the control valve CV sharply varies, the controlled flow value hardly varies.
The pressure-type flow control device FCS itself is known, and therefore a detailed description thereof is omitted herein.
However, in this kind of the pressure-type flow control device FCS, the orifice OL having a minute hole diameter is employed, and therefore secular changes in the hole diameter of the orifice OL is inevitable. The changes in the hole diameter produce a difference between the set flow rate (i.e., controlled flow rate value) of the pressure-type flow control device FCS and the actual flow rate value of the gas Go which actually flows through the orifice OL. Moreover, in order to detect the difference, so-called flow monitoring needs to be frequently performed, which poses a problem that the operability of a semiconductor manufacturing apparatus, the quality of a manufactured semiconductor, and the like are seriously affected.
Therefore, in the field of the pressure-type flow control device, a measure has been taken heretofore which detects the changes in the hole diameter of the orifice OL as soon as possible at an early stage to thereby prevent the generation of the difference between the controlled flow rate value obtained by the pressure-type flow control device FCS and the actual flow rate value of the gas Go which actually flows through the orifice. For the detection of the changes in the hole diameter of the orifice OL of this kind and the like, gas flow measuring methods employing a so-called build-up system or build-down system have been used in many cases
On the other hand, the gas flow measurement employing the build-up system or the build-down system requires temporarily stopping of the supply of actual gas, and thus poses a problem that the gas flow measurement reduces the operating ratio of a semiconductor manufacturing apparatus and gives great influence on the quality and the like of a manufactured semiconductor.
Therefore, in recent years, a development of a flow control device equipped with flow monitor which enables simple real-time monitoring about whether or not the flow control of supply gas is appropriately performed without temporarily stopping the supply of actual gas has been advanced in the field of the flow control device of this kind.
For example, FIG. 20 shows an example thereof, in which a flow control device equipped with flow monitor 20 is configured from a flow passage 23, a first pressure sensor 27a which detects the inlet side pressure, an opening/closing control valve 24, a thermal type mass flow sensor 25, a second pressure sensor 27b, a narrowed portion (sonic nozzle) 26, a calculation control unit 28a, an input/output circuit 28b, and the like.
The thermal type mass flow sensor 25 has a flow straightening body 25a, a branch flow passage 25b which branches the flow rate of a predetermined ratio F/A from the flow passage 23, and a sensor body 25c provided in the branch flow passage 25b, in which a flow rate signal Sf which shows the total flow rate F is output to the calculation control unit 28a. 
The narrowed portion 26 is a sonic nozzle which passes a fluid of a flow rate proportional to the upstream side pressure when a pressure difference between the upstream side pressure and the downstream side pressure exceeds a predetermined value (i.e., in the case of fluid flow under a critical condition). In FIG. 20, SPa and SPb denote pressure signals, Pa and Pb denote pressures, F denotes the total flow rate, Sf denotes a flow rate signal, and Cp denotes a valve opening degree control signal.
The calculation control unit 28a feeds back the pressure signals Spa and Spb from the pressure sensors 27a and 27b and the flow rate signal Sf from the flow sensor 25, and then outputs the valve opening degree control signal Cp to thereby feedback-control the opening/closing control valve 24. More specifically, a flow rate setting signal Fs from the input/output circuit 28b is input into the calculation control unit 28a, so that the flow rate F of a fluid which flows into the flow control device 20 is adjusted to be a flow rate set by the flow rate setting signal Fs.
Specifically, the calculation control unit 28a feedback-controls the opening and closing of the opening/closing control valve 24 using an output (pressure signal Spb) of the second pressure sensor 27b to thereby control the flow rate F of a fluid which flows through the sonic nozzle 26 and also the flow rate F with which the fluid actually flows is measured using an output (flow rate signal Sf) of the thermal type mass flow rate sensor 25 at this time, whereby an operation of the flow control device 20 is checked.
As described above, in the flow control device equipped with flow monitor 20 of FIG. 20, two kinds of systems of the pressure-type flow control of adjusting the opening degree of the opening/closing control valve 24 using the pressure signal Spb of the second pressure sensor 27b and the flow measurement using the thermal type mass flow sensor 25 of monitoring the actual flow rate are built in the calculation control unit 28a. Therefore, the flow control device equipped with flow monitor 20 enables simple and secure real-time monitoring about whether or not a fluid of the controlled flow rate corresponding to the set flow rate Fs actually flows, i.e., whether or not there is a difference between the controlled flow rate and the actual flow rate, and thus demonstrates high practical effects.
However, the flow rate control device equipped with flow monitor 20 of FIG. 20 still have a large number of problems to be solved.
As a first problem, when a difference is generated between a monitored flow rate value (actual flow rate value) and a controlled flow rate value, the generation of the difference can be detected by an alarm and the like but the controlled flow rate value cannot be automatically corrected, i.e., the set flow rate value Fs cannot be adjusted. Therefore, when the correction of the controlled flow rate value is delayed due to a certain cause, for example, the absence of an operation staff or the like, supply of gas of a flow rate different from the set flow rate value (gas of the actual flow rate) is continued, which produces various inconveniences in semiconductor manufacturing.
As a second problem, since two different kinds of measurement systems of the pressure-type flow measurement using the second pressure sensor 27b for controlling the flow rate and the flow measurement using the thermal type mass flow sensor 25 for monitoring the flow rate are built in, the structure of the flow control device equipped with flow monitor 20 is complicated and a reduction in size of the device and a reduction in the manufacturing cost cannot be achieved.
As a third problem, the flow control device equipped with flow monitor 20 is configured so that the calculation control unit 28a controls the opening/closing of the opening/closing control valve 24 using both the signals of the output Spb of the second pressure sensor 27b and the flow rate output Sf of the thermal type mass flow sensor 25 and also corrects the flow rate output Sf of the thermal type mass flow sensor 25 using the output Spa of the first pressure sensor 27a and that the opening/closing of the opening/closing control valve 24 is controlled using three signals of the two pressure signals of the first pressure sensor 27a and the second pressure sensor 27b and the flow rate signal from the thermal type mass flow sensor 25. Therefore, there are problems that the configuration of the calculation control unit 28a is complicated and also a stable flow control characteristic and outstanding high responsiveness as the pressure-type flow control device FCS are conversely reduced.